I believe these people that had extended layovers (stopovers) in Hong Kong probably built the flight using 2 one-way flights. I have often used this technique when traveling.
Traditionally, it is done by going to your favorite flight search engine then searching for one-way flights to the stopover city and then from stopover city to final destination. Often times the traveler will search multiple options for stopovers instead of only choosing one ahead of time in order to find the best value. As a stopover is a bonus to a already planned trip, the destination of the stopover can be any place the traveler has not seen before. The primary benefits of this technique are that the traveler can see more of the world, and often times save money while doing it.
I used this technique so often, and knew the tedious and time-consuming method was something that could be automated. So, my co-founder and I spent the last 1-1/2 years developing QuestOrganizer.com - The First Stopover Flight Deal Engine. QuestOrganizer does all the time consuming work for you by finding the optimal stopovers between any two destinations then easily organizing them to create a multiple one-way flight itenerary.